1. Field of the Invention
The present invention relates to a technique for correcting an image to be formed by a plurality of image forming units that has mis-registration in a sub-scanning direction.
2. Description of the Related Art
As an image recording method used for a color image forming apparatus such as a color printer and a color copy machine, an electrophotographic type is known. The electrophotographic type forms a latent image on a photosensitive drum using a laser beam, and develops the latent image with charged color materials (hereinbelow, referred to as “toner”). An image developed by toner is transferred onto a transfer sheet and fixed thereon to record the image.
In recent years, to form the image by the color image forming apparatus of the electrophotographic type at high speed, the color image forming apparatuses of the tandem type have been increasingly produced that include the same number of image forming units (developing devices and photosensitive drums) as those of toner colors, and subsequently transfer the images having different colors onto an image conveying belt or a recording medium.
In general, in the image forming apparatus, the image forming units corresponding to respective four process colors of cyan (C), magenta (M), yellow (Y), and black (K) are sequentially disposed in order of transfer. It is known that such a color image forming apparatus of a tandem type causes mis-registration because of a plurality of factors, and thus various methods for dealing with those factors are discussed.
One of the factors is inconsistency of a lens of an exposure device for irradiating the photosensitive drum with the laser beam, mis-positioning of mounting the lens, and mis-positioning of mounting the exposure device into a main body of the color image forming apparatus. The mis-positioning causes tilt or skew of an exposure scanning line, and a degree of the skew (hereinbelow, referred to as a “profile”) varies depending on the color, which may cause mis-registration. Further, the characteristic of profile is different depending on each image forming apparatus, recording engine, and also color.
As a method for dealing with such mis-registration, Japanese Patent Application Laid-Open No. 2007-3896 discusses a method for measuring a size of the tilt and the skew of the exposure scanning line using an optical sensor, correcting image data to offset the influence of the tilt and the skew, and then forming the corrected image. Since this method performs electrical correction to process the image data, a mechanical adjustment member and an adjusting procedure for assembly are not necessary. Accordingly, the color image forming apparatus can be reduced in size, and the mis-registration can be dealt with at low cost.
In electrically correcting the mis-registration (registration correction), correction is performed in a unit of one pixel and that in less than one pixel. In correction of a pixel unit, according to an amount of correction of the tilt and the skew of the exposure scanning line, the pixel of the image data is offset in unit of one pixel in a sub-scanning direction. In the descriptions hereinbelow, the processing of the offset is referred to as “transition processing”, and a position for performing the offset is referred to as a “transition point”.
In correction for less than one pixel, gradation values of the pixels on both sides of a boundary of the transition point in the image data are each adjusted based on gradation values of pixels on both sides thereof in the sub-scanning direction. By performing the correction in less than one pixel, an unnatural gap at the boundary of the transition point incurred by the correction of a pixel unit is solved, thereby creating a smooth image. The processing for correcting less than one pixel is referred to as “interpolation processing”.
When the color image forming apparatus performs printing, a special color may be specified for an object as a corporate color to print the object having the special color. This printing can be realized by specifying, when giving a print command, an absolute color appearing after the object is printed.
Specification of this color is referred to as “special-color specification”, and the object for which the special color is specified is referred to as a special-color object. The absolute color is, for example, a value defined by the Commission International de L'eclairage (CIE) and can be expressed in a color space of L*a*b* that is independent from a device.
In the correction of a pixel unit described above (transition processing), each color includes a different transition point. Therefore, for example, when a certain component color includes a transition point on an image rendered by mixing color materials of two or more colors, difference in the mixed color is incurred near the boundary of the transition point as illustrated in FIG. 13.
FIG. 13 illustrates a case where there is a transition point 1401 of cyan on an object in blue composed of cyan and magenta, and an upward triangle illustrated at an upper portion of the transition point 1401 indicates that the cyan is offset by one pixel in a direction of the upward triangle. When there is the transition point on the object specified with the special color as described above, it may cause a problem in that color unevenness is incurred near the boundary of the transition point.